To handle the increase in Internet traffic in recent years, digital coherent communication has begun to be introduced to long-distance optical communication, and the volume of communication has been progressively increasing. The next target of the digital coherence communication is middle-distance optical communication, and there have been strong demands for optical transmission-reception devices to be smaller in size and power consumption.
LN (lithium niobate) modulators are mainly used as modulators for current digital coherent communication transmitters. The size of current digital coherent transmission-reception modules is 5 inches×7 inches, which is a size large enough to accommodate an LN modulator. However, the size of the CFP2 standard, which has been studied for its introduction to middle-distance optical communication in the future, is 80 mm×40 mm, which is approximately 1/7 of the current size, and about 80 mm×20 mm, which is ½ of that size, is roughly the size of a transmission module. In practice, with the presence of a control board and the routing of fibers taken into account, the size of a transmission module is required to be kept to about 25 mm×20 mm. For this reason, a transmission-reception module of the CFP2 size cannot incorporate an LN modulator that has a size of the order of several cm.
In view of this circumstance, semiconductor modulators, which are easier to reduce the size than LN modulators, have been studied for their use as modulators for transmission modules. For example, an attempt has been started to employ a semiconductor modulator as a 100-Gbit/sec optical component to maintain the size of a transmission-reception package within the CFP2 standard.
The following is an example of related: Japanese Patent Application Laid-Open No. 2005-128440.
Meanwhile, a termination board is essential for an optical module including an optical element configured to be driven by a high-frequency signal such as a semiconductor modulator (hereinafter, referred to as “high-frequency optical element”). However, the termination board includes electronic components such as capacitors, termination resistors, and bias electrodes, and therefore needs to have a size of up to about 10 mm×mm. Hence, for a transmission module of the CFP2 standard or a smaller standard, the termination board can possibly be a factor that impedes reduction in size of the transmission module. Moreover, similar problems in size reduction also exist for various types of optical modules incorporating high-frequency optical elements other than transmission modules as described above.